Active material, method of manufacturing active material, and lithium-ion secondary battery

ABSTRACT

The present invention provides an active material which can increase the discharge capacity of a lithium-ion secondary battery as compared with the case using conventional LiMnPO 4  as a positive electrode active material. The active material in accordance with the present invention contains a crystallite of LiMnPO 4 , the crystallite having a size of 20 to 93 nm in a direction perpendicular to a (060) plane thereof.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active material, a method ofmanufacturing an active material, and Lithium-Ion Secondary Battery.

2. Related Background Art

Materials having a general formula of Li_(x)A_(y)PO₄ (where A is Cr, Mn,Fe, Co, Ni, Cu, or the like, 0<x<2, and 0<y<1) with an olivine structurecan reversibly intercalate and deintercalate lithium ions and thusfunction as a positive electrode active material of lithium-ionsecondary batteries. Since Li_(x)A_(y)PO₄ is superior to other activematerials in terms of safety, its practical utilization has been studied(see Japanese Patent Application Laid-Open Nos. 2004-259470 and2004-79276).

Using LiMnPO₄, in particular in Li_(x)A_(y)PO₄, as a positive electrodeactive material yields a high discharge voltage of about 4.1 V versusLi/Li⁺, whereby a high energy density can be expected (see the followingprior art literatures).

Japanese Patent Application Laid-Open Nos. 2006-40640 and 2007-119304.

Solution for next generation Lithium batteries. [online]. High PowerLithium Corporation, 2009. [retrieved on 2009-02-18]. Retrieved from theInternet:<URL:http://www.highpowerlithium.com/index.php?option=com_content&task=view&id=26&Itemid=57>.

High Performance nano-sized LiMnPO₄ systhesised via a Polyol method.[online]. High Power Lithium Corporation, 2008. [retrieved on2009-02-18]. Retrieved from the Internet:<URL:hftp://www.highpowerlithium.com/images/stories/HPL_presentation/hpl%20poster%20imlb%202008.pdf>.

Advance in Li Ion cathodes for HEV: Lithium Manganese Phosphate.[online]. High Power Lithium Corporation, 2008. [retrieved on2009-02-18]. Retrieved from the Internet:<URL:http://www.highpowerlithium.com/images/stories/HPL_presentation/hpl%20llibta%20presentation%202008.pdf>.

Manganese Phosphate: new high-Voltage Li-ion cathode. [online]. HighPower Lithium Corporation, 2007. [retrieved on 2009-02-18]. Retrievedfrom the Internet:<URL:http://www.highpowerlithium.com/images/stories/HPL_presentation/hpl_llibta_(—)2007.pdf>.

Electrochemical and Solid-State Letters, 5(6) A135-A137 (2002)

Journal of The Electrochemical Society, 156(2) A79-A83 (2009)

SUMMARY OF THE INVENTION

Even when LiMnPO₄ described in the above-mentioned literatures is usedas a positive electrode active material, however, a lithium-ionsecondary battery having a discharge capacity large enough for practicaluse has been hard to achieve.

In view of the problems of the prior art mentioned above, it is anobject of the present invention to provide an active material which canincrease the discharge capacity of a lithium-ion secondary battery ascompared with the case using conventional LiMnPO₄ as a positiveelectrode active material, a method of manufacturing the activematerial, and lithium-ion secondary battery.

For achieving the above-mentioned object, the active material inaccordance with the present invention contains a crystallite of LiMnPO₄,the crystallite having a size of 20 to 93 nm in a directionperpendicular to a (060) plane thereof. The lithium-ion secondarybattery in accordance with the present invention comprises a positiveelectrode, the positive electrode has a positive current collector and apositive active material layer disposed on the positive currentcollector, the positive active material layer contains the activematerial in accordance with the present invention.

By using the active material in accordance with the present invention asa positive electrode active material, a lithium-ion secondary batterycan increase its discharge capacity as compared with the case usingconventional LiMnPO₄.

Preferably, in the active material in accordance with the presentinvention, the crystallite has a size of 75 to 210 nm in a directionperpendicular to a (210) plane thereof.

This makes it easier for the lithium-ion secondary battery to increasethe discharge capacity remarkably.

The method of manufacturing an active material in accordance with thepresent invention comprises a hydrothermal synthesis step of irradiatinga mixture containing a lithium source, a phosphate source, a manganesesource, and water and having a pH of 7 to 9 with an electromagneticwave, so as to heat the mixture under pressure such that the mixturereaches a crystal growth temperature T of 180° C. or higher.

The method of manufacturing an active material in accordance with thepresent invention directly heats the solvent and solute of the mixtureby irradiation with an electromagnetic wave instead of conventionalexternal heat sources such as thermostatic baths and heating furnaces,and thus can promote the generation and crystal growth of LiMnPO₄ in themixture. By stopping the irradiation with the electromagnetic wave, themethod of manufacturing an active material in accordance with thepresent invention can cool the mixture more rapidly than methods usingthe conventional external heat sources, and thus is easier to stop thecrystal growth of LiMnPO₄. Therefore, the method of manufacturing anactive material in accordance with the present invention can finelydivide the crystallite of LiMnPO₄, so as to control the size of thecrystallite such that it falls within the range of 20 to 93 nm in adirection perpendicular to a (060) plane thereof.

Preferably, in the method of manufacturing an active material inaccordance with the present invention, the crystal growth temperature Tis 190 to 240° C. in the hydrothermal synthesis step.

By using the active material obtained at the crystal growth temperatureT to 190 to 240° C. as a positive electrode active material, thelithium-ion secondary battery can remarkably increase the dischargecapacity.

Preferably, in the method of manufacturing an active material inaccordance with the present invention, the mixture is caused to reachthe crystal growth temperature T at a heating rate of 5 to 50° C./min inthe hydrothermal synthesis step.

By using the active material obtained at the heating rate of 5 to 50°C./min as a positive electrode active material, the lithium-ionsecondary battery can remarkably increase the discharge capacity.

Preferably, after reaching the crystal growth temperature T, the mixtureis held at the crystal growth temperature T for 300 min or less byirradiating the mixture with the electromagnetic wave in thehydrothermal synthesis step.

By using thus obtained active material as a positive electrode activematerial, the lithium-ion secondary battery can remarkably increase thedischarge capacity.

The present invention can provide an active material which can increasethe discharge capacity of a lithium-ion secondary battery as comparedwith the case using conventional LiMnPO₄ as a positive electrode activematerial, a method of manufacturing the active material, and alithium-ion secondary battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph of the active material of Example 22 in thepresent invention captured through a scanning electron microscope (SEM).

FIG. 2 is a photograph of the active material of Example 26 in thepresent invention captured through the scanning electron microscope(SEM).

FIG. 3 is a schematic sectional view of a lithium-ion secondary batterycomprising a positive active material layer containing an activematerial in accordance with an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Active Material

The active material in accordance with an embodiment of the presentinvention is constituted by a crystallite of LiMnPO₄, while thecrystallite has a size of 20 to 93 nm in a direction perpendicular to a(060) plane thereof. The “direction perpendicular to a (060) plane ofthe crystallite” will be referred to as “(060)-plane direction”hereinafter.

The “size of the crystallite” is an average value of crystallite sizescalculated according to the following Scherrer equation (1) from resultsof measurement by X-ray diffractometry (XRD):

D _(hkl)=(K·λ)(β cos θ)  (1)

In equation (1), D_(hkl) is the “size of the crystallite” in a directionperpendicular to an (hkl) plane of the crystallite, K is the Scherrerfactor, λ is the wavelength of the X-ray used for the XRD, β is thespread (half width or integral width) of a diffracted X-ray peak, and θis the Bragg angle of the diffracted X-ray. In the following, the sizeof the crystallite in the (060) plane may also be abbreviated as “D₀₆₀”when appropriate.

In the crystallite of LiMnPO₄, lithium ions (Li⁺ ions) are retainedalong the (060)-plane direction, so that a conduction path of lithiumions is formed along the (060)-plane direction. The crystallite attainsits optimal structure when its size in the (060) plane-direction is 20to 93 nm. A lithium-ion secondary battery using the crystallite havingthus optimized structure as a positive electrode active material yieldsa discharge capacity greater than that in the case using conventionalLiMnPO₄.

When D₀₆₀ is less than 20 nm, the amount of lithium ions retained by thecrystallite becomes smaller, thereby making it harder to increase thedischarge capacity of the lithium-ion secondary battery. As D₀₆₀increases beyond 70 nm, the lithium ion conductivity tends to decreasegradually in the crystallite. When D₀₆₀ exceeds 93 nm, the lithium ionconductivity remarkably decreases, whereby the discharge capacity of thelithium-ion secondary battery is harder to increase.

D₀₆₀ is preferably 20 to 70 nm, more preferably 53 to 57 nm. In thiscase, the discharge capacity of the lithium-ion secondary battery tendsto increase remarkably.

The size of the crystallite in a direction perpendicular to a (210)plane thereof is preferably 75 to 210 nm, more preferably 140 to 150 nm.In the following, the size of the crystallite in a directionperpendicular to a (210) plane thereof may be abbreviated as “D₂₁₀” whenappropriate.

When D₂₁₀ falls within the above-mentioned range, the discharge capacityof the lithium-ion secondary battery tends to increase remarkably ascompared with the case where D₂₁₀ is outside of the range. Though therelationship between D₂₁₀ and the discharge capacity is not completelyclear, the inventors think that the form of the (060) plane of thecrystallite is easier to deform when D₂₁₀ is outside of theabove-mentioned range than when not, whereby the crystallite tends tolose its lithium ion retention or conductivity, thus lowering thedischarge capacity.

Lithium-Ion Secondary Battery

As illustrated in FIG. 3, The lithium-ion secondary battery 100 inaccordance with this embodiment is equipped with a power generatingelement 30 comprising a planar positive electrode 10 and a planarnegative electrode 20 opposing each other and a planar separator 18disposed between and adjacent to the positive and negative electrodes;an electrolyte containing lithium ions; a case 50 accommodating them ina closed state; a negative electrode lead 62 having one end partelectrically connected to the negative electrode 20 and the other endpart projecting out of the case 50; and a positive electrode lead 60having one end part electrically connected to the positive electrode 10and the other end part projecting out of the case 50.

The negative electrode 20 has a negative electrode current collector 22and a negative electrode active material layer 24 formed on the negativeelectrode current collector 22. The positive electrode 10 has a positiveelectrode current collector 12 and a positive electrode active materiallayer 14 formed on the positive electrode current collector 12. Theseparator 18 is placed between the negative electrode active materiallayer 24 and positive electrode active material layer 14.

The positive electrode active material layer 14 contains theabove-mentioned active material in accordance with this embodiment.

Method of Manufacturing Active Material

The method of manufacturing an active material in accordance with anembodiment of the present invention will now be explained. The method ofmanufacturing an active material in accordance with this embodimentcomprises a hydrothermal synthesis step of irradiating a mixturecontaining a lithium source, a phosphate source, a manganese source, andwater and having a pH of 7 to 9 with an electromagnetic wave, so as toheat the mixture under pressure such that the mixture reaches a crystalgrowth temperature T of 180° C. or higher. The method of manufacturingan active material in accordance with this embodiment can manufacturethe active material in accordance with the previous embodiment.

Hydrothermal Synthesis Step

First, in the hydrothermal synthesis step, the above-mentioned lithiumsource, phosphate source, manganese source, and water are put into areaction vessel, so as to prepare a mixture (aqueous solution) havingthem dispersed therein. For preparing the mixture, a mixture of thephosphate source, manganese source, and water may be refluxed at firstbefore adding the lithium source thereto, for example. The reflux canform a complex of the phosphate and manganese sources.

Any reaction vessel may be used as long as it can closely seal theinside thereof and is resistant to heat and pressure. The reactionvessel is constituted by a material having a property of transmittingtherethrough an electromagnetic wave such as a microwave or carbondioxide laser, which will be explained later, without absorbing it. Inthis embodiment, a reaction vessel made of a fluororesin such aspolytetrafluoroethylene may be used, for example.

The pH of the mixture is adjusted to 7 to 9. D₀₆₀ of the active materialbecomes greater than 93 nm and smaller than 20 nm when the pH of themixture is too small and too large, respectively.

While various methods can be employed for adjusting the pH of themixture to 7 to 9, adding an acidic or basic reagent to the mixture ispreferred. Hydrochloric acid or the like may be used as the acidicreagent, while aqueous ammonia solution or the like may be used as thebasic reagent. The amount of the acidic or basic reagent to be added maybe adjusted appropriately according to the amount of the mixture and thekinds and compounding ratios of the lithium, phosphate, and manganesesources.

As the lithium source, at least one species selected from the groupconsisting of LiNO₃, Li₂CO₃, LiOH, LiCl, Li₂SO₄, and CH₃COOLi may beused, for example.

As the phosphate source, at least one species selected from the groupconsisting of H₃PO₄, NH₄H₂PO₄, (NH₄)₂HPO₄, and Li₃PO₄ may be used, forexample.

As the manganese source, at least one species selected from the groupconsisting of MnSO₄.5H₂O, MnCO₃, and Mn(CH₃COO)₂.4H₂O may be used, forexample.

Two or more kinds of lithium sources, two or more kinds of phosphatesources, and two or more kinds of manganese sources may be usedtogether.

Next, the reaction vessel is closed, and the mixture within the reactionvessel begins to be irradiated with an electromagnetic wave. Theelectromagnetic wave passes through the reaction vessel, so as to reachthe mixture. The electromagnetic wave causes an oscillatingelectromagnetic field which stimulates water, the lithium source, thephosphate source, or the manganese source to rotate or vibrate, therebyheating the mixture. This generates a vapor within the reaction vessel,which increases the pressure within the reaction vessel, therebypressurizing the mixture. The mixture is continuously irradiated withthe electromagnetic wave at least until the mixture reaches the crystalgrowth temperature T. As a consequence, the hydrothermal synthesis ofthe above-mentioned active material in accordance with this embodimentproceeds in the mixture.

Unlike methods indirectly heating the mixture within the reaction vesselby using external heat sources such as thermostatic baths and heatingfurnaces, the method of manufacturing an active material in accordancewith this embodiment directly heats the mixture with an electromagneticwave whose output and irradiation time can be adjusted freely.Therefore, as compared with the cases using external heat sources, thisembodiment can substantially neglect influences of the thermalconduction and convection, so as to make it possible to heat and coolthe mixture rapidly and uniformly and easy to control the temperature ofthe mixture. By heating the mixture rapidly and uniformly, thisembodiment can promote the hydrothermal synthesis and crystal growth ofLiMnPO₄. Cooling the mixture rapidly and uniformly can suppressexcessive crystal growth, thereby making it possible to control thecrystallite of LiMnPO₄ such that its D₀₆₀ falls within the range of 20to 93 nm. Heating the mixture within the reaction vessel by using theconventional thermostat bath or heating furnace is harder to control thetemperature of the mixture than the present invention, whereby thecrystallite of LiMnPO₄ may grow in excess so that its D₀₆₀ exceeds 93nm, for example.

Any electromagnetic wave may be used as long as it has a frequency whichpasses through the reaction vessel and stimulates at least one of waterand the lithium, phosphate, and manganese sources in the mixture torotate or vibrate. Specific examples of the electromagnetic wave includemicrowaves, lasers, and infrared rays, among which a microwave or laseris preferably used. The microwave may have a frequency of 2.45 GHz or915 MHz, for example. The microwave having such a frequency is suitablefor heating water in particular. As the laser, a carbon dioxide laser(CO₂ laser) may be used, for example. The mixture within the reactionvessel may be irradiated with the electromagnetic wave by using acommercially available electromagnetic irradiation apparatus such as amicrowave irradiation apparatus or carbon dioxide laser irradiationapparatus.

In the hydrothermal synthesis step, the crystal growth temperature T isadjusted to 180° C. or higher. Preferably, the crystal growthtemperature T is controlled so as to fall within the range of 190 to240° C. When the crystal growth temperature T is lower than 180° C., theactive material is easier to lower its crystallinity, whereby its D₀₆₀may be less than 20 nm. As a result, the discharge capacity of thelithium-ion secondary battery becomes smaller than that in the casewhere the crystal growth temperature T falls within the range mentionedabove. When the crystal growth temperature T is too high, D₀₆₀ tends tobecome so large that the discharge capacity of the lithium-ion secondarybattery may decrease.

Preferably, the mixture is caused to reach the crystal growthtemperature T at a heating rate of 5 to 50° C./min in the hydrothermalsynthesis step. When the heating rate is outside of the range of 5 to50° C./min, the discharge capacity of the lithium-ion secondary batterytends to become lower than that in the case where the heating rate fallswithin the range of 5 to 50° C./min. Using a pulsed wave as theelectromagnetic wave makes it easier to control the heating rate.

Preferably, after reaching the crystal growth temperature T, the mixtureis held at the crystal growth temperature T for 300 min or less byirradiating the mixture with the electromagnetic wave. In other words,it will be preferred if the irradiation of the mixture with theelectromagnetic wave is stopped within 300 min from the time when themixture reaches the crystal growth temperature T by irradiating themixture with the electromagnetic wave. The time elapsed since themixture reached the crystal growth temperature T by irradiating themixture with the electromagnetic wave until the irradiation of themixture held at the crystal growth temperature T with theelectromagnetic wave is stopped will be referred to as “time t” in thefollowing.

When the time t is longer than 300 min, D₀₆₀ tends to become greaterthan that in the case where the time t is 300 min or less, therebydecreasing the discharge capacity.

The time t may be 0 min. That is, the irradiation of the mixture withthe electromagnetic wave may be stopped at the time when the mixturereaches the crystal growth temperature T. For example, “0 min” can bedefined as the time elapsed since a CPU determined that the mixturereached the crystal growth temperature T in an electromagneticirradiation apparatus for controlling the temperature of the mixture byusing the CPU until an output power supply for the electromagnetic waveis actually turned off in response to an instruction to turn off theoutput power supply from the CPU.

The pressure applied to the mixture (the internal pressure of thereaction vessel) in the hydrothermal step, which is uniquely determinedby the temperature of the aqueous solution (mixture), is preferablyadjusted to 0.2 to 6 MPa. When the pressure applied to the mixture istoo low, the finally obtained active material tends to lower itscrystallinity, thereby reducing the capacity density of the activematerial and the discharge capacity of the lithium-ion secondarybattery. When the pressure applied to the mixture is too high, thereaction vessel tends to require excessive resistance to pressure,thereby increasing the cost for manufacturing the active material. Thesetendencies can be suppressed when the pressure applied to the mixturefalls within the range mentioned above. The pressure within the reactionvessel can be measured by a pressure sensor provided with thecommercially available electromagnetic irradiation apparatus andcontrolled by an automatic pressure control system, for example.

Heat-Treatment Step

Preferably, the mixture after the hydrothermal synthesis step isheat-treated in this embodiment. This can promote the reaction of thelithium, phosphate, and manganese sources that failed to react in thehydrothermal synthesis step and the crystal growth of LiMnPO₄ that hasnot grown the crystal sufficiently.

The heat-treatment temperature is preferably 400 to 800° C. When theheat-treatment temperature is too low, the crystal growth of LiMnPO₄tends to become insufficient, thereby lowering the capacity density ofthe active material. When the heat-treatment temperature is too high,the crystal growth of LiMnPO₄ tends to proceed in excess, so as toincrease D₀₆₀, thereby making it harder to increase the dischargecapacity of the lithium-ion secondary battery. These tendencies can besuppressed when the heat-treatment temperature falls within the rangementioned above.

Preferably, the heat-treatment time for the mixture is 0.5 to 20 hr.Preferably, the mixture is heat-treated in a nitrogen, argon, air, orvacuum atmosphere.

The mixture obtained by the hydrothermal synthesis step may be heatedfor about 1 to 30 hr at about 60 to 150° C. before being heat-treated.This heating removes impurities such as surplus moisture and organicsolvents from the mixture, thereby turning the mixture into a drypowder. Heat-treating the dried mixture can prevent the active materialfrom taking impurities therein and homogenize particle forms.

Preferably, a carbon source or carbon particle is added to the mixtureafter the hydrothermal synthesis step before the heat treatment. Thisallows at least a part of the active material surface to carry thecarbon material. As a result, the electrical conductivity of thusobtained active material can be improved. The carbon source or carbonparticle may be added before the hydrothermal synthesis step.

Examples of substances constituting the carbon particle includeactivated carbon, carbon black, graphite, hard carbon, and soft carbon,among which carbon black is preferably used. This makes it easier toimprove the electrical conductivity of the active material. Usingacetylene black as carbon black makes it easier to improve theelectrical conductivity of the active material.

While preferred embodiments of the active material and method ofmanufacturing the active material have been explained in the foregoing,the present invention is not limited thereto.

For example, the active material of the present invention can also beused as an electrode material for electrochemical devices other than thelithium-ion secondary battery. Examples of such electrochemical devicesinclude secondary batteries other than the lithium-ion secondarybattery, e.g., metallic lithium secondary batteries (using an electrodecontaining the active material of the present invention as an cathodeand a metallic lithium or a lithium alloy such as lithium aluminum as ananode), and electrochemical capacitors such as lithium capacitors. Theseelectrochemical devices can be used for power supplies forself-propelled micromachines, IC cards, and the like and decentralizedpower supplies placed on or within printed boards.

EXAMPLES

The present invention will now be explained more specifically withreference to examples and comparative examples, but will not be limitedto the following examples.

Example 1 Hydrothermal Synthesis Step

LiOH.H₂O, (NH₄)₂HPO₄, and MnSO₄.5H₂O were dissolved in water and mixed,so as to prepare an aqueous solution. The respective concentrations ofLiOH.H₂O, (NH₄)₂HPO₄, and MnSO₄.5H₂O were adjusted to 0.3 M, 0.1 M, and0.1 M. Water used was not subjected to any deaeration operation such asremoval of dissolved oxygen in particular. The pH of the aqueoussolution was 9.5. Concentrated hydrochloric acid was added dropwise tothe aqueous solution, so as to adjust the pH of the aqueous solution to8.0. Subsequently, the aqueous solution was left for 2 days in the air.Oxygen in the aqueous solution and oxygen in the air were presumed tohave oxidized Mn²⁺ in the solution gradually during the 2 days.

After being left for the 2 days, the aqueous solution was sealed closelyin a pressure-resistant vessel made of polytetrafluoroethylene. Then,using a commercially available microwave irradiation apparatus, theaqueous solution within the pressure-resistant vessel began to beirradiated with a microwave. The irradiation with the microwavegenerated steam within the pressure-resistant vessel, so as topressurize the aqueous solution and cause it to reach a crystal growthtemperature T. The microwave had the maximum output of 1000 W and afrequency of 2.45 GHz. The crystal growth temperature T was adjusted to190° C. The microwave was emitted in pulses. The hydrothermal synthesisstep using the microwave will be referred to as “microwave hydrothermalsynthesis step” hereinafter.

In the microwave hydrothermal synthesis step, the heating rate at whichthe aqueous solution was caused to reach the crystal growth temperatureT was adjusted to 10° C./min. The time t elapsed since the aqueoussolution was caused to reach the crystal growth temperature T by beingirradiated with the microwave until the irradiation of the aqueoussolution with the electromagnetic wave was stopped after continuouslyholding the aqueous solution at the crystal growth temperature T was 5min. That is, the microwave irradiation was stopped after the aqueoussolution having reached the crystal growth temperature T wascontinuously kept at the crystal growth temperature T for 5 min.

After stopping the irradiation with the microwave, the aqueous solutionwas naturally cooled to 160° C. in the microwave irradiation apparatus.After being naturally cooled, the pressure-resistant vessel was takenout of the apparatus and cooled with water.

Analysis by Powder X-Ray Diffractometry

The liquid taken out of the water-cooled pressure-resistant vessel wasfiltered and washed with water, so as to yield the active material ofExample 1, which was then dried at 80° C. The dried active material wasanalyzed by the powder X-ray diffractometry (XRD). As a result of theanalysis, it was seen that LiMnPO₄ was generated alone by the microwavehydrothermal synthesis step in Example 1. D₀₆₀ and D₂₁© of LiMnPO₄ werealso measured. Table 1 represents the results.

Heat-Treatment Step

For 1 hr, LiMnPO₄ of Example 1 obtained by the hydrothermal synthesisstep and carbon black (manufactured by Denki Kagaku Kogyo K.K. under theproduct name of DAB-50) were subjected to alternating steps of mixingand milling them at 550 rpm for 1 min and stopping mixing andpulverizing them for 1 min, so as to prepare a positive electrodematerial. The mixing ratio between LiMnPO₄ and carbon black was adjustedsuch that LiMnPO₄:carbon black=80 parts by mass:10 parts by mass. Formixing and milling, a planetary ball mill manufactured by Retsch GmbH(type: PM-100) was used. Zirconia balls were used as media for mixingand milling. The mixed and milled positive electrode material washeat-treated in an argon gas flow. In the heat treatment, thetemperature of the positive electrode material was raised to 700° C. in1 hr, held at 700° C. for 1 hr, and then naturally cooled to roomtemperature.

Making of a Half Cell

A positive electrode coating material was prepared by adding 90 parts bymass of the heat-treated positive electrode material and 10 parts bymass of PVDF (polyvinylidene fluoride) to NMP (N-methyl-2-pyrrolidone).The ratios of LiMnPO₄, carbon black, and PVDF, which were solids in thepositive electrode coating material, were adjusted such thatLiMnPO₄:carbon black:PVDF=80 parts by mass:10 parts by mass:10 parts bymass.

The positive electrode coating material was applied to an aluminum foilhaving a thickness of 20 μm. The applied positive electrode coatingmaterial was dried and then pressed under pressure, so as to yield apositive electrode. Subsequently, a Li foil was cut into a predeterminedsize, which was then attached to a copper foil (having a thickness of 15μm), so as to yield a negative electrode. The positive and negativeelectrodes were laminated with a separator made of a macroporouspolyethylene film interposed therebetween, so as to yield a multilayerbody (matrix). As outer lead terminals, an aluminum foil (4 mm (W)×40 mm(L)×80 μm (T)) and a nickel foil (4 mm (W)×40 mm (L)×80 μm (T)) wereultrasonically welded to the positive and negative electrodes,respectively. Polypropylene (PP) having grafted with maleic anhydridebeforehand was wound about and thermally bonded to each of the outerlead terminals. This aims to improve the sealability between each outerterminal and an outer package. Prepared as the outer package of thebattery was one made of an aluminum laminate material having a structureof PET(12)/Al(40)/PP(50). PET and PP refer to polyethyleneterephthalateand polypropylene, respectively. The parenthesized numbers representthicknesses of their corresponding layers. Here, an envelope was madesuch that PP faces the inside.

Thus obtained multilayer body was put into the outer package of thebattery, 1-M LiPF₆/EC+DEC (with a volume ratio of 30:70) was injectedtherein, and the outer package of the battery was heat-sealed in vacuum,so as to make an electrode evaluation half cell of Example 1.

Measurement of the Discharge Capacity

Using the half cell of Example 1, the discharge capacity (unit: mAh/g)at a discharge rate of 0.1 C (the current value by whichconstant-current discharging completed in 10 hr) was measured. Table 1represents the measured results. The discharge capacity represented inTable 1 was the discharge capacity per gram of the active material. Inthe measurement, assuming that LiMnPO₄ as the positive electrode activematerial had a nominal capacity of 171 mAh/g, the charging anddischarging was carried out at 0.1 C. The upper charge voltage and thelower discharge voltage were set to 4.5 V (vs. Li/Li⁺) and 2.0 V (vs.Li/Li⁺), respectively. The charging was effected until the positiveelectrode reached the upper charge voltage and the charge currentdecayed to 1/20 C. The measurement temperature was 25° C.

Examples 2 to 24 and 27 to 34

The active materials and half cells of Examples 2 to 24 and 27 to 34were made as in Example 1 except that the crystal growth temperature T,heating rate, and time t in the microwave hydrothermal synthesis stepwere adjusted to their corresponding values listed in Table 1. As aresult of the analysis by XRD, it was seen that the microwavehydrothermal synthesis step generated LiMnPO₄ alone as an activematerial in each of Examples 2 to 24 and 27 to 34, too. FIG. 1illustrates a photograph of LiMnPO₄ of Example 22 taken through an SEM.

Example 25

In Example 25, the microwave hydrothermal synthesis step was carried outwith the aqueous solution whose pH was adjusted to 7.0 by addingconcentrated hydrochloric acid dropwise thereto. The crystal growthtemperature T, heating rate, and time t in the microwave hydrothermalsynthesis step in Example 25 were adjusted to their corresponding valueslisted in Table 1. Except for these matters, the active material andhalf cell of Example 25 were made as in Example 1. As a result of theanalysis by XRD, it was seen that the microwave hydrothermal synthesisstep generated LiMnPO₄ alone as an active material in Example 25, too.

Examples 26 and 36

In Examples 26 and 36, the microwave hydrothermal synthesis step wascarried out with the aqueous solution whose pH was adjusted to 9.0 byadding concentrated hydrochloric acid dropwise thereto. The crystalgrowth temperature T, heating rate, and time t in the microwavehydrothermal synthesis step in Examples 26 and 36 were adjusted to theircorresponding values listed in Table 1. Except for these matters, theactive materials and half cells of Examples 26 and 36 were made as inExample 1. As a result of the analysis by XRD, it was seen that themicrowave hydrothermal synthesis step generated LiMnPO₄ alone as anactive material in Examples 26 and 36, too. FIG. 2 illustrates aphotograph of LiMnPO₄ of Example 26 taken through the SEM.

Example 35

In the hydrothermal synthesis step, Example 35 used a carbon dioxide gaslaser instead of the microwave (1 kW at CW), and an autoclave made ofstainless steel instead of the pressure-resistant vessel made ofpolytetrafluoroethylene. The crystal growth temperature T, heating rate,and time t in the microwave hydrothermal synthesis step in Example 35were adjusted to their corresponding values listed in Table 1. Exceptfor these matters, the active material and half cell of Example 35 weremade as in Example 1. As a result of the analysis by XRD, it was seenthat the microwave hydrothermal synthesis step generated LiMnPO₄ aloneas an active material in Example 35, too.

Comparative Example 1

In Comparative Example 1, the microwave hydrothermal synthesis step wascarried out with the aqueous solution whose pH was adjusted to 6.5 byadding concentrated hydrochloric acid dropwise thereto. The crystalgrowth temperature T, heating rate, and time t in the microwavehydrothermal synthesis step in Comparative Example 1 were adjusted totheir corresponding values listed in Table 1. Except for these matters,the active material and half cell of Comparative Example 1 were made asin Example 1. As a result of the analysis by XRD, it was seen thatMn₅(PO₃(OH))₂(PO₄)₂(H₂O)₄ was generated as an impurity in addition toLiMnPO₄ in the active material of Comparative Example 1.

Comparative Example 2

In Comparative Example 2, no concentrated hydrochloric acid was addeddropwise to the aqueous solution before carrying out the microwavehydrothermal synthesis step. That is, Comparative Example 2 carried outthe microwave hydrothermal synthesis step by using the aqueous solutionwhose pH was 9.5. The crystal growth temperature T, heating rate, andtime t in the microwave hydrothermal synthesis step in ComparativeExample 2 were adjusted to their corresponding values listed in Table 1.Except for these matters, the active material and half cell ofComparative Example 2 were made as in Example 1. As a result of theanalysis by XRD, it was seen that Mn₅(PO₃(OH))₂(PO₄)₂(H₂O)₄ andLi_(0.48)Mn_(0.89)O₂ were generated as impurities in addition to LiMnPO₄in the active material of Comparative Example 2.

Comparative Example 3

Comparative Example 3 carried out the hydrothermal synthesis step byheating an autoclave made of stainless steel closely sealing the aqueoussolution therein in a thermostat bath. The crystal growth temperature T,heating rate, and time t in the hydrothermal synthesis step inComparative Example 3 were adjusted to their corresponding values listedin Table 1. Except for these matters, the active material and half cellof Comparative Example 3 were made as in Example 1. As a result of theanalysis by XRD, it was seen that the microwave hydrothermal synthesisstep generated LiMnPO₄ alone as an active material in ComparativeExample 3, too.

Comparative Example 4

The active material and half cell of Comparative Example 4 were made asin Example 1 except that the crystal growth temperature T in themicrowave hydrothermal synthesis step was adjusted to the valuerepresented in Table 1. As a result of the analysis by XRD, it was seenthat the microwave hydrothermal synthesis step generated LiMnPO₄ aloneas an active material in Comparative Example 4, too.

D₀₆₀ and D₂₁₀ of LiMnPO₄ and the discharge capacity per gram of theactive material were determined in each of Examples 2 to 36 andComparative Examples 1 to 4 as in Example 1. Table 1 lists the results.

TABLE 1 Crystal Heating Time Discharge growth temp. rate t D₀₆₀ D₂₁₀capacity pH Heating means T(° C.) (° C./min) (min) (nm) (nm) (mAh/g)Example 1 8.0 microwave 190 10 5 20 132 132 Example 2 8.0 microwave 19010 0 20 128 130 Example 3 8.0 microwave 190 10 60 20 133 135 Example 48.0 microwave 190 10 300 21 136 136 Example 5 8.0 microwave 200 10 0 30131 132 Example 6 8.0 microwave 200 10 5 31 133 134 Example 7 8.0microwave 200 10 60 33 136 134 Example 8 8.0 microwave 200 10 300 35 140135 Example 9 8.0 microwave 210 10 0 39 135 133 Example 10 8.0 microwave210 10 5 40 137 134 Example 11 8.0 microwave 210 10 60 42 139 135Example 12 8.0 microwave 210 10 300 44 143 136 Example 13 8.0 microwave220 10 0 50 138 135 Example 14 8.0 microwave 220 10 5 52 140 136 Example15 8.0 microwave 220 10 60 53 143 137 Example 16 8.0 microwave 220 10300 56 148 136 Example 17 8.0 microwave 230 10 0 52 140 138 Example 188.0 microwave 230 10 5 53 141 139 Example 19 8.0 microwave 230 10 60 53145 140 Example 20 8.0 microwave 230 10 300 55 147 140 Example 21 8.0microwave 240 10 0 53 142 140 Example 22 8.0 microwave 240 10 5 54 144142 Example 23 8.0 microwave 240 10 60 54 146 143 Example 24 8.0microwave 240 10 300 57 150 143 Example 25 7.0 microwave 220 10 60 90200 130 Example 26 9.0 microwave 220 10 60 20 80 135 Example 27 8.0microwave 180 10 300 20 75 120 Example 28 8.0 microwave 250 10 0 93 205123 Example 29 8.0 microwave 220 10 315 91 210 121 Example 30 8.0microwave 220 3 5 55 215 110 Example 31 8.0 microwave 220 5 5 53 172 121Example 32 8.0 microwave 220 20 5 50 138 131 Example 33 8.0 microwave220 50 5 50 135 124 Example 34 8.0 microwave 220 54 5 48 121 115 Example35 8.0 CO₂ laser 220 10 60 50 140 135 Example 36 9.0 microwave 180 50300 20 70 110 Comparative Example 1 6.5 microwave 220 10 60 101 228 100Comparative Example 2 9.5 microwave 220 10 60 10 58 105 ComparativeExample 3 8.0 thermostat bath 220 — 60 200 300 80 Comparative Example 48.0 microwave 170 10 0 15 100 100

As Table 1 represents, by irradiating the aqueous solution having a pHof 7 to 9 with the microwave or carbon dioxide laser, each of Examples 1to 36 carried out the hydrothermal synthesis step of heating the aqueoussolution under pressure such that the aqueous solution reached thecrystal growth temperature T of 180° C. or higher. As a result, it wasseen that Examples 1 to 36 yielded the active materials each containinga crystallite of LiMnPO₄ and exhibiting D₀₆₀ of 20 to 93 nm in LiMnPO₄by the hydrothermal synthesis step.

In Examples 1 to 36 each containing a crystallite of LiMnPO₄ andexhibiting D₀₆₀ of 20 to 93 nm in LiMnPO₄, the discharge capacity pergram of the active material was seen to be greater than that in any ofthe comparative examples.

Comparative Example 1 carrying out the microwave hydrothermal synthesisstep by using the aqueous solution having a pH of less than 7 was seento yield D₀₆₀ greater than 93 nm and a discharge capacity smaller thanthat in any of the examples.

Comparative Example 2 carrying out the microwave hydrothermal synthesisstep by using the aqueous solution having a pH exceeding 9 was seen toyield D₀₆₀ falling short of 20 nm and a discharge capacity smaller thanthat in any of the examples.

Comparative Example 3 carrying out the hydrothermal synthesis step byheating the autoclave made of stainless steel closely sealing theaqueous solution therein in the thermostat bath instead of using themicrowave was seen to yield D₀₆₀ greater than 93 nm and a dischargecapacity smaller than that in any of the examples.

Comparative Example 4 setting the crystal growth temperature T in themicrowave hydrothermal synthesis step to less than 180° C. was seen toyield D₀₆₀ falling short of 20 nm and a discharge capacity smaller thanthat in any of the examples.

Comparing Examples 4 and 27 with each other and Examples 21 and 28 witheach other proved that adjusting the crystal growth temperature T to 190to 240° C. yielded the discharge capacity greater than that in the casewhere the crystal growth temperature T was outside of the range of 190to 240° C.

Comparing Examples 30 to 34 with each other proved that the heating rateof 5 to 50° C./min yielded the discharge capacity greater than that inthe case where the heating rate was outside of the range of 5 to 50°C./min.

Comparing Examples 16 and 29 with each other proved that the time t of300 min or less reduced D₀₆₀ and increased the discharge capacity ascompared with the case where the time t was longer than 300 min.

REFERENCE SIGNS LIST

-   -   10 . . . positive electrode; 20 . . . negative electrode; 12 . .        . positive electrode current collector; 14 . . . positive        electrode active material layer; 18 . . . separator; 22 . . .        negative electrode current collector; 24 . . . negative        electrode active material layer; 30 . . . multilayer body; 50 .        . . case; 60, 62 . . . lead; 100 . . . lithium-ion secondary        battery

1. An active material containing a crystallite of LiMnPO₄, thecrystallite having a size of 20 to 93 nm in a direction perpendicular toa (060) plane thereof.
 2. An active material according to claim 1,wherein the crystallite has a size of 75 to 210 nm in a directionperpendicular to a (210) plane thereof.
 3. A lithium-ion secondarybattery comprising a positive electrode; wherein the positive electrodehas a positive current collector and a positive active material layerdisposed on the positive current collector; wherein the positive activematerial layer contains the active material according to claim
 1. 4. Amethod of manufacturing an active material, the method comprising ahydrothermal synthesis step of irradiating a mixture containing alithium source, a phosphate source, a manganese source, and water andhaving a pH of 7 to 9 with an electromagnetic wave, so as to heat themixture under pressure such that the mixture reaches a crystal growthtemperature T of 180° C. or higher.
 5. A method of manufacturing anactive material according to claim 4, wherein the crystal growthtemperature T is 190 to 240° C. in the hydrothermal synthesis step.
 6. Amethod of manufacturing an active material according to claim 4, whereinthe mixture is caused to reach the crystal growth temperature T at aheating rate of 5 to 50° C./min in the hydrothermal synthesis step.
 7. Amethod of manufacturing an active material according to claim 4,wherein, after reaching the crystal growth temperature T, the mixture isheld at the crystal growth temperature T for 300 min or less byirradiating the mixture with the electromagnetic wave in thehydrothermal synthesis step.